Herpesvirus saimiri was naturally transmitted from squirrel monkeys excreting the virus to one of two owl monkeys housed in the same cage. The owl monkey became infected approximately three months after contact was initiated. H. saimiri was consistently isolated from the peripheral lymphocytes until this animal died eight months later. During this period the owl monkey developed specific antibody to H. saimiri to a maximal neutralization index of 5.5 logs. The other monkey remained uninfected for an ovservation period of one year. The documentation of this horizontal transmission of H. saimiri infection from squirrel monkeys to an owl monkey suggests that owl monkeys developing spontaneous malignant lymphomas associated with H. saimiri infection may also have acquired the infection in this manner.